Rejuvenation will most likely be your top heal and fights where you do need the HPS you will benefit more from the 4 pc set bonus than from 100-200 in stats gained. In cases where you believe RJ will mainly overheal, you will be probably be better off using WG and holding on casting the RJ.
Please note that a 50%+ overheal is not necessarily to be a shamed of due to the nature of aggressive healing during progression and the weakness in how absorbs interact with healing.
I would suggest using T14 4 piece (if you have them), with the talents SOTF and HOTW. The rought throughput and reduced rejuvations cost is a lot better compared to the T15 set bonus.
I raid in a hardcore PvE Guild, and I first switchted from T14 -> T15 when I had 4/5-T15HC tokens.
You might not want to wait as long, depending on your Guild progress rate. But I would suggest passing T15 for Dps classes / other healers. As pretty much everybody benefit more than us with the tier bonuses.
I pretty much got rid of T14 bonus when we were 11/13 HC progress
I was using x2 normal pieces and x2 hc pieces of T14 until that point.
Read your post incorrectly thought it was T14 v T15..
You should keep your current 4set over ilvl.
If you have T14 switch back to it! Use WG and swiftmend synced on CD. Get yourself the epic meta gem. And wreck the healing with lowest ilvl!